(1) Field of the Invention
The present invention relates to a method of and an apparatus for controlling supercharge pressure for a turbocharger.
(2) Description of the Prior Art
A turbocharger is constructed in such a manner that an exhaust turbine is rotated by exhaust gas at high temperature and high pressure so that the pressure within an engine intake manifold connected to the outlet of the turbocharger increases above atmospheric pressure as the number of rotations of a compressor in the turbocharger increases. As a result, the supply of a large quantity of intake air to the engine becomes possible by the supercharge pressure thus obtained, with the result that high engine torque, and high engine output can be obtained and an improvement of the engine fuel consumption can be obtained as well.
On the other hand, however, in the high load operating zone, the turbine speed becomes too high to control the supercharge pressure. Accordingly, provision is normally made for an exhaust bypass valve downstream of the turbine so as to discharge the pressure at the turbine inlet and to prevent the supercharge pressure from exceeding an allowable maximum pressure.
The control of the supercharge pressure by the exhaust bypass valve is as follows; a diaphragm actuator is provided so as to drive the exhaust bypass valve, with the supercharge pressure produced downstream of the compressor being the operating pressure, and the supercharge pressure is maintained at a preset supercharge pressure, i.e., it is maintained constant by controlling the duty value of an electromagnetic valve which discharges the operating pressure to the atmosphere. For instance, when the actual supercharge pressure exceeds a preset supercharge pressure, the pressure exerted on a diaphragm for reducing the duty valve of the electromagnetic valve increases. As a result, a rod coupled to the diaphragm is moved against a spring thereof by the diaphragm and the movement of the rod permits the exhaust bypass valve to operate in the direction of its opening through a likage mechanism. An increase in the proportion of opening the valve enables the exhaust gas amount bypassing the turbine to increase, thereby lowering the engine speed and this in turn reduces the supercharge pressure to the preset supercharge pressure.
Since a large engine output is required when accelerating, the actual supercharge pressure is sometimes increased provisionally above the preset supercharge pressure during normal operation. Normally, when an acceleration condition is detected, a control unit increases the duty value of the electromagnetic valve and the operation pressure acting on the diaphragm is discharged, so as to reduce the pressure exerted on the diaphragm. As a result, the opening of the exhaust bypass valve is delayed, thus increasing the supercharge pressure due to the increased exhaust gas flow to the turbine (See Japanese Patent Disclosure Specification No. 58-180726/1983).
In a turbocharger in which acceleration correction is carried out according to the prior art as described above, the duty value of the electromagnetic valve is increased by predetermined amounts and there is a possibility that the supercharge pressure sometimes deviates from the optimum value due to a variation in the elastic force of the springs of the diaphragm or due to a change in the engine performance with the elapse of time. This causes a condition where the supercharge pressure is excessively low and a desirable acceleration cannot be obtained, while, when the supercharge pressure is excessively high, it will bring about engine knocking.